Chemistry Reference
In-Depth Information
8
Foams and Liquid Aerosols
It was pointed out earlier that foams and emulsions are related in that they represent
a physical state in which one fluid phase is finely dispersed in a second phase, and
that the state of dispersion and the long-term stability (persistence) normally are
dependent on the presence of one or more additives that alter the energy of the
interface between the two phases. In emulsions, as each phase is a liquid, such fac-
tors as the solubility of additives in each phase must be considered. In foams, one
phase (the dispersed phase) is a gas, so problems related to transfer of materials
from the continuous to the dispersed phase effectively do not exist.
Liquid aerosols are, of course, the inverse of foams—liquid drops dispersed in a
gas. However, the ''nature of the beast'' in the case of aerosols pretty much pre-
cludes any significant surfactant effects beyond that of affecting the particle size
produced at the atomization stage. Some aspects of liquid aerosol technology are
presented below. For the most part, however, this chapter addresses the basic role
of surfactants in the formation and stabilization of foams, and gives some leads as
to surfactant properties that may be useful for the suppression or elimination of
foams where their presence would be considered detrimental.
The presence of foam in a product or process may or may not be desirable.
Foams have wide technical importance in such fields as firefighting, in polymeric
foams and foam rubbers, in foamed structural materials such as concrete, and,
of course, in a myriad of food products. They also have certain aesthetic utility
in many detergent and personal care products, although their presence may not
add much to the overall effectiveness of the process. Foams also serve useful pur-
poses in industrial processes such as mineral separation (froth flotation) and for
environmental reasons for the suppression of liquid and fume emissions in some
processes such as electroplating. In the latter case, the presence of a foam blanket
over the electroplating solution helps prevent solution splattering and the loss of
volatile materials, therefore reducing the costs of maintaining an acceptable working
environment.
Unwanted foams may be a significant problem in many technical processes,
including sewage treatment, coatings applications, and crude oil processing. By
understanding the physical and chemical characteristics of materials that produce
and sustain foams, it becomes easier to identify ways to counteract or overcome
those foaming tendencies. The following sections cover some of the basic physical
